1. Field of the Invention
The present invention relates to semiconductor laser and beam splitting devices, and optical recording/reproducing, optical communication, and optomagnetic recording/reproducing apparatuses using the semiconductor laser and beam splitting devices, the optomagnetic recording/reproducing apparatus being adapted to optomagnetically record or reproduce information.
2. Related Background Art
In recent years, remarkable technological developments have been made in the fields of optical communication and optical information processing, and the most advanced variable wavelength performance has become increasingly important as a function required in semiconductor lasers in wavelength multiplex transmission and frequency modulation transmission methods for widening the transmission bands in communication fields so as to achieve higher-density recording by wavelength multiplexing in the field of optical recording. In these technical fields, the density of information is increased in terms of time and space by utilizing multiplexing and parallel characteristics of light wavelengths. The present applicant has proposed the following semiconductor laser and its driving method so as to achieve most advanced variable wavelength control. More specifically, as disclosed in Japanese Laid-Open Patent Application No. 63-211787 (corresponding U.S. Pat. No. 4,982,408), an active layer comprises a plurality of quantum well layers having different quantum levels, and the magnitude of injection currents into these quantum well layers are changed to variably control an oscillation frequency.
Technological advances in an optical information recording/reproducing apparatus for recording or reproducing information using such a semiconductor laser have been achieved to a remarkable extent. In particular, a recording/reproducing apparatus using an optomagnetic recording medium or a phase change recording medium is expected to be the most promising future apparatus because it can erase or update information and the recording capacity can be made larger. In such a recording/reproducing apparatus, studies on "overwrite" operations of information to increase a data transfer speed, "direct verifying" operations of recorded information, and "parallel recording/reproduction" (reproduction simultaneously with recording) using a plurality of light spots have been extensively made.
Overwrite methods for optomagnetic recording media are mainly classified into a magnetic field modulation method and an optical modulation method. The magnetic field modulation method is a method for applying a bias magnetic field modulated in accordance with recording information while a recording medium is being irradiated with a laser beam having a predetermined intensity. On the other hand, the optical modulation method is a method for generating a laser beam intensity-modulated in accordance with recording information on a recording medium to record the information while a predetermined magnetic field is being applied to the recording medium. In recent years, another overwrite method is proposed in which a recording medium is irradiated with a pulsed laser beam while an A.C. magnetic field having a predetermined frequency is being applied to the recording medium. FIGS. 1A to 1F are timing charts for explaining a recording operation using the above recording method. FIG. 1A shows a reference clock, FIG. 1B shows an A.C. magnetic field, and FIG. 1C shows a pulsed laser beam. In this recording method, the A.C. magnetic field synchronized with the reference clock is applied to the recording medium, and a laser beam is projected on the recording medium in synchronism with positive and negative peak positions of this A.C. magnetic field. In this case, the laser beam is projected at the positive or negative peak position of the A.C. magnetic field on the basis of a signal modulated with a recording signal shown in FIG. 1D. By these operations, the direction of magnetization of a temperature rise portion of the recording medium is aligned with a direction corresponding to the recording signal, thereby forming the domains shown in FIG. 1E. The shape of each domain formed is a shape of an arrow feather, as shown in FIG. 1F. Note that an overwrite method for generating light beams having high and low powers modulated in accordance with recording information is available as an overwrite method for a phase change recording medium.
On the other hand, as a direct verifying scheme, for example, two light spots are formed on the same information track at two adjacent timings, information is recorded with the leading light spot, and the recorded information is reproduced with the trailing beam spot, thereby performing a verifying operation. The two beam spots may be formed using one optical head or two optical heads. The parallel recording/reproducing methods include a method (ISOM '91, Lecture No. 3G-1) using an image rotating element (image rotator) to scan the target tracks with four light spots and a method for recording information in a plurality of information tracks with an A.C. magnetic field and light pulses, as proposed in Japanese Laid-Open Patent Application No. 4-291045.
In the above conventional direct verifying method, when two light spots, i.e., the recording and reproduction light spots, are to be formed, two separate semiconductor lasers are used as light sources or a semiconductor laser array is used to form the recording and reproduction light spots. For this reason, it is very difficult to adjust the positions of the two light spots, i.e., an optical system. When the semiconductor laser array is used, an element distance cannot be detected due to thermal interference between the semiconductor laser elements. For this reason, the two light spots cannot be set closer to each other, and sufficient performance of the optical system cannot be obtained, resulting in inconvenience.
In the parallel recording or recording using a light pulse modulated with a magnetic field, the recorded information must be reproduced and verified, thus prolonging the recording time. In addition, in parallel recording and reproduction using a plurality of light spots, a stable tracking control scheme has not been yet established. In addition, in direct verifying upon radiation of recording and verifying light spots on the corresponding information tracks, any tracking control scheme for stably scanning one information track with two light spots has not been yet established.
In the direct verifying operation using two light spots formed by one optical head, the curvature of an inner track of the information recording disk is different from that of an outer track thereof, tracking accuracy varies depending on the position on the disk, and verifying accuracy is also degraded depending on the position on the disk. In the formation of two light spots by two optical heads, since the two optical heads radiate linearly polarized light components on the disk, these light components are susceptible to the influence of the birefringence of the disk substrate. In addition, the light spots must be accurately radially moved on the disk in the radial seek operation, thereby limiting structural freedom in design and hence keeping an optical system from being made compactly.